Breathing mask



FIG. 1 is a perspective view of the breathing mask;

FIG. 2 is a left side view of the breathing mask;

FIG. 3 is a top side view of the breathing mask;

FIG. 4 is a right side view of the breathing mask; and,

FIG. 5 is a front view of the breathing mask.

The broken lines illustrate environmental structure of the breathing mask, which forms no part of the claimed design. 

The ornamental design for a breathing mask, as shown and described. 